Commercially available vinyl ethers are produced by means of the base-catalysed addition of acetyl groups onto alcohols under pressure. The compounds obtained contain the structural element H2C═CH—OR and have been used technically for many years. These compounds enjoy particular attention in the context of cationic and photocationic polymerisation, because they generally represent very reactive compounds as a result of the number of electrons of their double bond.
However, users continually complain that volatile components with an intense odour are formed during cross-linking, and that in higher concentrations, these are irritants and therefore unsafe with reference to industrial hygiene. For reasons of industrial health and safety, comprehensive precautionary measures are therefore necessary. These not only represent a considerable financial expense for the user, but also increase the cost of the products.
For some time, it has been known that one of the principal components of these undesirable, volatile by-products is acetaldehyde. This occurs in a subsidiary reaction of the vinyl ether with the atmospheric moisture. T. MORIGUCHI et al. describe one possible reaction pathway in Macromolecules 1995, 28, 4334-4339.
Various approaches to solving this problem have been under discussion for a considerable time. From an economic perspective, the rearrangement of readily accessible allylethers to isopropenylether using noble metal catalysts seems most promising (J. V. CRIVELLO, U.S. Pat. No. 5,486,545 of Jan. 23, 1996). However, this view overlooks the fact that, like the commercial vinyl ethers, isopropenylether can also enter a subsidiary reaction with water during the cationic and photocationic polymerisation, leading to the formation of propionaldehyde. The demand for an emission-free cross-linking cannot therefore be fulfilled with isopropenylether. In principle, open-chain vinyl ethers are not capable of achieving this because in this case, volatile decomposition products can always be formed in the presence of moisture.
However, cyclic vinyl ethers, such as 2,3-dihydrofuranes and 2,3-dihydropyranes are almost ideal vinyl ethers. During photocationic reaction, they are indeed also capable of entering subsidiary reactions with water, but they do not produce volatile decomposition products because the irritant aldehyde component remains firmly anchored in the molecule. However, the accessibility of these heterocyclic compounds—especially the derivatives with two or more dihydrofurane or dihydropyrane groups which are suitable for cross-linking—is extremely difficult and expensive with regard to the synthesis. As a result, the synthesis of larger quantities has so far not been technically possible on a cost-favourable scale.
By contrast, the class of 4-methylene-1,3-dioxolanes is substantially more accessible. Initial attempts to cross-link 4-methylene-1,3-dioxolanes are described in the U.S. Pat. No. 2,445,733 of Jul. 7, 1945. Depending on the metal ion involved, the Friedel-Crafts catalysts used in this context lead to materials of a reddish-brown color and not to solvent-resistant networks. The use of a solution of zinc chloride in alcohol (H. ORTH, Angew. Chem. 1952, 64, 544-553) provided an improvement, but the polymerisations carried out were particularly exothermic and in some cases their course after addition of the catalyst was explosive. However, it must be noted on the positive side that the resulting networks provide considerable surface hardness and associated good processing properties.
More recently, it has become known that 4-methylene-1,3-dioxolanes also exhibit photocationic activity. For instance, K. D BELFIELD and F. B. ABDELRAZZAQ, Macromolecules 1997, 30, 6985-88, describe a photocationic cross-linking of 2,2′-(1,4-phenylene)-bis-(4-methylene-1,3-dioxolane) with 2-phenyl-4-methylene-1,3-dioxolane. However, both monomers are of an aromatic nature, i.e. they have aromatic substituents in the 2-position. Now, however, it is known that 4-methylene-1,3-dioxolanes with a 2,2-diphenyl- or 2-phenyl-2-alkyl substitution eliminate the ketone component during polymerisation (R. S. DAVIDSON, G. J. HOWGATE, J. Photochem. Photobiol. A, 1997, 109, 185-193 and Y. HIRAGURI, T. ENDO, J. Polym. Sci. Part A: Polym. Chem. 1989, 27, 4403-4411), i.e. more or less volatile components are split off. The requirement for emission-free cross-linking can therefore not be fulfilled.
The polymerisation processes provided in the state of the art operate either in a solvent or in a large composition. The duration of the polymerisation is in the range of several hours and the yields are not quantitative. They cannot therefore simply be transferred to photocationic cross-linking especially of films and thin layers.